GEOMORPHOLOGY OF THE ROSILLOS MOUNTAINS, BREWSTER COUNTY, TEXAS

Formed during the Tertiary, the Rosillos Mountains, Brewster County, Texas, are an elliptical Quartz Syenite laccolith. Surrounding the laccolith is an elevated pediment surface composed of remnants of a once very active depositional alluvial fan complex. Analysis of fan sediments suggests varied depositional processes, ranging from large, viscous debris flows, transporting one-meter size boulders, to sheetflows and streamflows, which have created a complex of at least eight coalescing alluvial fans. Using Christenson and Purcell's (1985) general characteristics for the determination of the age of alluvial fans, including drainage patterns, depth of incision, surface morphology, preservation of fan surface, desert pavement and desert varnish development, and B-horizon and calcic horizon development, nearly all the fans qualify as 'old' alluvial fan deposits. The age of the fans makes morphometric measurements difficult, but not impossible. The basin-to-fan-area relationship exhibits a positive correlation, with an exponent of 0.47 suggesting negative allometry and a disproportionate contribution of sediment from the catchment to the fan areas. Paleohydrologic evidence from Tornillo Creek and recent field mapping by Dennie (2001) suggests two drops in local baselevel. Three distinct paleo-Tornillo creek terrace levels exist above the modern day Tornillo Creek. The highest abandoned gravels, located 200 feet above the modern day creek level, have large cross-stratification, and crosscut the toeslopes of Cottonwood and Star Creek alluvial fans. Changes in local baselevel attribute to the multiple depositional episodes encompassing the southern and western alluvial fans. Sombrero Peak Ranch catchment basin renders two distinct alluvial fans exiting from its' evacuation point. Differentiation of the two fans was determined by studying topographic maps and remote sensing coverage for the area. Direction and pattern of drainage of the two fans provide further evidence of a baselevel shift during the unroofing of the Rosillos laccolith.